Last week, Human Rights Watch called for their resignation, saying they had "failed to conduct genuine, impartial and effective investigations".

The group said that the tribunal had been "subject to frequent politically motivated interference" from the ruling party, many of whom were former Khmer Rouge leaders.

Prime Minister Hun Sen - at one point a mid-ranking Khmer Rouge commander - as well as his information minister and foreign minister have all separately made it clear in recent months that they do not want further prosecutions after the second trial has concluded.

It said that while he was not influenced by political statements, "his ability to withstand such pressure by government officials to perform his duties independently, could always be called in doubt, and this would also call in doubt the integrity of the whole proceedings" of the two new cases.

Judge Blunk has been a controversial figure since he took over from French judge Marcel Lemonde to investigate cases three and four, the BBC's Guy De Launey in Phnom Penh says.

Many of his own international staff have been so dismayed by the way the crimes were being investigated that they quit in protest. One called Mr Blunk's office "dysfunctional", our correspondent says.